The present invention relates generally to the measurement of magnetic fields, and more specifically to a borehole sensor having multiple coils for sensing horizontal field components.
In sub-surface geophysical exploration, a probe having sensors for measuring various physical parameters at different depths is lowered down a borehole. One type of measurement uses a magnetic field sensor to measure induced magnetic fields and thereby glean information regarding the possible presence of conductive ore bodies. Associated with the magnetic field sensor is a transmitter coil which, when energized, establishes a magnetic field. The field induces electrical currents in conductive regions in the vicinity, and the induced currents establish a magnetic field which is measured. By performing such measurements at various depths, it is possible to establish field profiles.
In certain applications, the transmitter coil is a large horizontal loop of wire which is laid out on the surface of the earth in the general vicinity of the borehole location. In other applications, the transmitter coil is a small diameter coil that is lowered into the borehole, possibly being physically located in the same housing as the sensor.
U.K. Published Patent Application No. 2,148,012 to Cobcroft discloses a borehole probe for measuring magnetic field components along the axis of the borehole and in the two transverse directions. Due to a limitation on the probe diameter, the transverse field components are measured by an array of short coils. The reference shows short coils for the axial coordinate, but a single long coil could also be used for the measurement along the borehole axis. The transmitter coil is on the earth's surface. U.S. Pat. No. 4,636,731 to Savage et al. discloses a similar arrangement with transmitter coils located within the borehole probe itself.
In operation, the induced fields penetrate the regions within the coils, and induce voltages proportional to the rate of change of the magnetic field (dB/dt). It is, however, known to provide a direct measurement of magnetic field by use of a feedback coil associated with and magnetically coupled to the detector's main coil. This technique is described by G. Clerc and D. Gilbert in "La cotre-reaction de flux appliquee aux bobines a noyau magnetique utilisee pour l'enregistrement des variations rapides du champ magnetique." Extract des Annales de Geophysique, Tome 20, n4 (October-December 1964). Briefly, the main coil is coupled to the input terminal of the amplifier, and an output signal is applied to the feedback coil which operates to cancel the magnetic field through the main coil. Thus, the main coil becomes a null detector and the feedback current is linearly proportional to the magnetic field.